On Pyroligncoiis JEthcr. 315 



pui'pose : which latter, however, renders the observation of the 

 entrance of a star very uncertain and troublesome." 



" M. Frauenhofer conceived the fortunate idea of placing, in 

 the focus of the telescope, a glais pierced in the centre with a 

 hole sufficient to admit of a metal ring, which was attached 

 thereto, and which ring was then turned perfectly circular: 

 and, as it was never afterwards separated from the glass, its 

 circular form remained uninjured." 



" The great advantage attending this method is that the 

 metal circle requires no other support, since the glass itself is 

 fixed to the diaphragm : and thus the double concentric circle 

 (which foi'ms the micrometer) seems to hang without support 

 in the expanse of the heavens." 



[Our mathematical instrument-makers occasionally con- 

 struct the micrometer of a transit instrument by means of fine 

 lines drawn with a diamond on glass : and I conceive it would 

 not be more difficult to draw t'iXso concentric circles in the same 

 manner ; which would probably answer every purpose to which 

 the circular micrometer is usually applied. There appears to 

 be only one objection to this plan : which is, that the loss of 

 light (which however is not great) may prevent the observation 

 oi small comets, and veri/ minute stars ; to which class of objects 

 the circular micrometer is peculiarly adapted. — F. B.] 



LVIII. On Pyrolignemis uEthcr. By Philip Taylor, 



To the Editors ofihePhilosopJiical Magazine and Journal. 



Gentlemen, — In the Philosophical Magazine for August 

 last, I observed the following extract from one of the foreign 

 journals* : 



" On examining diffiirent samjiles of jiyroligneous acid, M, 

 Diibcreiner lately i'ound alcohol in two of them obtained from 

 birch-v.ood." 



As I believe there is an error in this statement, I shall trouble 

 you at this time with a few remarks upon it, and in a future 

 number of the Magazine detail more fully the experiments 

 which have led me to the conclusions 1 have Ibrmed on this 

 subject. 



The peculiar fluid which M. Diibcreiner calls alcohol was 

 first discovered hy mc in the year IKl'2; at whicii time I was 

 extensively engaged in the manufacture of pyroligneous acid 

 and the various preparations f()rmed with it. In attempting to 

 purily tills acid by a new process, I tibserved lliat both tiie co- 

 louring matter and that which gives it its characteristic otiour 



• Schweijrgei's N. J', fiir Cheiii. n. V\\yi, 



\\ Y 2 ^^ ere 



